1) Field of the Invention
The present invention relates to semiconductor processing equipment, more particularly, to a method and apparatus for reducing particle contamination in a semiconductor processing apparatus.
2) Discussion of Related Art
One type of processing apparatus for semiconductor wafers is a single wafer processor in which one wafer at a time is processed in a processing chamber. An example of a single wafer reactor is shown in FIG. 1. A susceptor 120 divides a chamber 112 into one portion which is below the susceptor (the lower portion) 124, and a second portion which is above the susceptor (the upper portion) 122. The susceptor 120 is generally mounted on a shaft 126 which rotates the susceptor about its center to achieve a more uniform processing of the wafer. A flow of a processing gas, such as a deposition gas 115, is provided in the upper portion 122 of the chamber. The chamber generally has a gas inlet passage 178 at one side thereof, and a gas exhaust passage 116 at an opposite side to achieve a flow of the processing gas across the wafer. The susceptor 120 is heated in order to heat the wafer to a desired processing temperature. One method used to heat the susceptor is by the use of lamps 134 provided around the chamber and directing their light into the chamber and onto the susceptor 120. In order to control the temperature to which the wafer is being heated, the temperature of the susceptor is constantly measured. This is often achieved by means of an infrared temperature sensor 136 which detects the infra-red radiation emitted from the heated susceptor.
A problem with this type of processing apparatus is that some of the processing gas, which is often a gas or mixture of gases for depositing a layer of a material on the surface of the wafer, tends to flow around the edge of the susceptor and deposits a layer of the material on the back surface of the susceptor. Since the deposited material is generally different from the material of the susceptor, the deposited layer has an emissivity which is different from that of the emissivity of the susceptor. Thus, once the layer of the material is deposited on the back surface of the susceptor, the infrared temperature sensor detects a change caused by the change in the emissivity of the surface from which the infra-red radiation is emitted. This change indicates a change in temperature of the susceptor which actually does not exist.
One technique which has been used to prevent the problem of deposits on the back surface of the susceptor is to provide a flow of an inert gas 121, such as hydrogen, into the lower portion of the chamber at a pressure slightly greater than that of the deposition gas in the upper portion of the chamber. One apparatus for achieving this is described in the commonly assigned application for U.S. Patent of Roger N. Anderson et al., Ser. No. 08/099/977, filed Jul. 30, 1993, entitled "Gas Inlets For Wafer Processing Chamber". Since the inert gas in the lower portion of the chamber is at a higher pressure, it will flow around the edge of the susceptor from the lower portion of the chamber and into the upper portion of the chamber. This flow of the inert gas prevents the flow of the deposition gas 115 into the lower portion of the chamber. Unfortunately, however, as the purge gas flows from the lower portion of the chamber to the upper portion of the chamber in order to exit through the exhaust passage 116 located in the upper portion 122 of chamber 122, it carries metal contaminants from the lower portion of the chamber into the upper portion, resulting in contamination of wafers being processed.
Another problem associated with the processing apparatus of FIG. 1 is that as deposition gas 115 exits the chamber through exhaust passage 116, the deposition gas cools and condenses to form deposits 114 within the exhaust passage 116. Deposition gas cools because the apparatus of FIG. 1 is a "cold wall reactor". That is, the sidewall of the deposition chamber is at a substantially lower temperature than is susceptor 120 (and wafer) during processing because the sidewall is not directly irradiated by lamp 134 due to reflectors 135 and because cooling fluid is circulated through the sidewall. Since the sidewall and the exhaust outlet passage are at a lower temperature, the deposition gas heated by susceptor 120 cools while in the passage and forms deposits 114 therein. These deposits 114 can find their way back into chamber 112 and onto the wafer being processed. Deposits 114 can detrimentally affect film quality and uniformity which can result in a substantial decrease in device yield.
Thus, what is desired is a method and apparatus which can reduce the formation of deposits in the exhaust passage and which can reduce metal contamination from the lower portion of the chamber.